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Creating custom views
1. The document discusses how to create a custom PieChart view in Android by extending the View class and implementing various callback methods like onDraw(), onMeasure(), and onTouchEvent(). 2. It demonstrates how to define attributes for the custom view, initialize attributes in the constructor, and read attribute values. 3. Optimizing techniques for custom views such as doing less work in onDraw(), using invalidation to update only dirty regions, and minimizing layout requests are also covered.
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Creating custom views
- 1.
- 2. A custom viewshould be... 2
- 3. A custom viewshould be... Conformity XMLify Accessibility Compatibility 3
- 4.
- 5. public class PieChartextends View { public PieChart(Context context, AttributeSet attrs) { super(context, attrs); init(context, attrs); } public PieChart(Context context) { super(context); } init(context, null); Constructors private void init(Context context, AttributeSet attrs) { } } 5
- 6. <resources> <declare-styleable name="PieChart"> <attr name="showText" format="boolean" /> <attr name="labelPosition" format="enum"> <enum name="left" value="0" /> <enum name="right" value="1" /> </attr> </declare-styleable> </resources> /res/values/attrs.xml 6
- 7. <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/androi d" xmlns:custom="http://schemas.android.com/apk/res/com.exa mple.customview"> <com.example.customview.PieChart custom:labelPosition="left" custom:showText="true" /> </LinearLayout> /res/layout/main.xml 7
- 8. private void init(Contextcontext, AttributeSet attrs) { TypedArray a = context.getTheme().obtainStyledAttributes(attrs, R.styleable.PieChart, 0, 0); try { mShowText = a.getBoolean(R.styleable.PieChart_showText, false); mTextPos = a.getInteger(R.styleable.PieChart_labelPosition, 0); } finally { a.recycle(); } } The author's initialization 8
- 9. private void init(Contextcontext, AttributeSet attrs) { if (attrs != null) { TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.PieChart); mShowText = a.getBoolean(R.styleable.PieChart_showText, false); mTextPos = a.getInteger(R.styleable.PieChart_labelPosition, 0); a.recycle(); } } My initialization 9
- 10. onDraw() 10
- 11. What to draw?Canvas How to draw? Paint 11
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- 13. @Override protected void onMeasure(intwidthMeasureSpec, int heightMeasureSpec) { int minw = getPaddingLeft() + getPaddingRight() + getSuggestedMinimumWidth(); int w = resolveSize(minw, widthMeasureSpec); int h = resolveSize(MeasureSpec.getSize(w), heightMeasureSpec); setMeasuredDimension(w, h); } MUST setMeasuredDimension 13
- 14. float start =0.f; for (int i = 0; i < series.size(); i++) { float sweep = (float) (series.get(i) / sum * 360.f); paint.setColor(Color.rgb(0xff, 0xff, 0xff)); canvas.drawArc(oval, start, sweep, true, paint); start += sweep; } onDraw() 14
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- 16. Interactive 16
- 17. View boolean onTouchEvent() ACTION_DOWN All TouchEvent: return true; onClick(): return false; 17
- 18. ViewGroup boolean onInterceptTouchEvent() ACTION_DOWN handle self: return true; pass to child: return false; 18
- 19. Derive from onTouchEvent() onClick(),onLongClick() onDown(), onFling() onLongPress,() onScroll() onShowPress(), onSingleTapUp() 19
- 20. Optimizing 20
- 21. Do Less, LessFrequently onDraw() invalidate() requestLayout() 21
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- 23.
- 24. requestLayout() if has complexUI custom ViewGroup 24
- 25. References Creating a ViewClass Custom Drawing Making the View Interactive Optimizing the View 25
- 26.